RPi Screw Connector Breakout Board

If there ever was a hardware project suitable for a noob this is probably it.

This Project comes from the inventiveness of Mike Cook and I have included links to his excellent support video.

This video shows the assembly of the protected version which uses surface mounted components.

I should say that when it says solderless it refers to any connections you make to the RPi (ribbon cable) and the breakout ports (screw terminals) the board its self will require some soldering (not a lot but some).

All the diagrams have been produced by Mike and are available for download (the link appears later on).

This solder-less screw terminal access break out board connects to the RPi through a ribbon cable and transfers the signals to labelled screw terminals.

There are three versions presented here. One that just transfers the data signals, and two that offer some form of protection for the processor’s GPIO lines.

It is suggested that if this is to be approached as a novice project then you should be aware that

The first basic unbuffered project is probably the easiest but this is the one that affords the lowest level of protection for your RPi.

The second the buffered version adds protection for your RPi this is small and compact.

The third option which Mike calls the through hole version is the phisicaly larger option this actually makes it easier for a novice to build it offers protection for your RPi and is probably the best option.

Overall none of these are big projects the materials are cheap and without a douubt this is the project that will allow you to to have a RasperryPi that will interface with the outside world.

We hope we will be able to follow this up with some simple projects to go with this board.

It is here you can get access to logic signals that provide inputs and outputs to the computer.

The P1 header is designed to connect to a 26 way two row 1.2xx mm pitch IDC connector.

IDC stands for Insulation Displacement Cable and is a way of making lots of connections with minimum effort.

It works by having ribbon cable, named after the first world war flaying ace Barron von Ribbon Cable, being cut into by a special connector.

This is quite a precise connector because it cuts through the cable’s insulation without cutting through the conductor.

Hence the name insulation displacement. It practice it is easy enough to construct a cable.

Simply slide the connector on the ribbon cable and apply pressure.

This can be from a purpose made press or if you are only doing a few you can squeeze it up in a vice.

You can get a cute set of pliers to do this job but to be honest a small bench vice will do a better job.

The trick is to get the cable carefully aligned, apply a bit of pressure, check the alignment again and finally tighten the vice.

Being careful not to crush the whole assembly.

Basic Breakout

This cable then connects to a 26 way latched header on the breakout board.

The board is built onto strip (veroboard) board and the header has 0.1” spacing.

This means you have to cut away a small slice of copper between the rows of holes to prevent each pair of connections shorting out.

You can do this by using a very sharp knife, I used a scalpel with replaceable blades.

The technique is simple, get a steel rule and score a line along the copper strips.

Then move the rule about 1mm and score again.

Then you remove the copper in between the two score lines with a scooping motion of the knife.

You get a nice clean break. Examine it carefully to make sure there are no whiskers of copper still joining the two sides.

Then you can solder the header onto the board and also the screw connectors.

Fig3

If you are using the basic, no protection circuit then that is about it.

All you need to do is to wire each one of the active pins through to a screw terminal connector.

You can do this basically in two ways.

First of you can arrange the screw connectors in any grouping you like and use flexible wire to connect up each terminal to each pin.

Or you can use tinned copper wire to make links.

By using horizontal links only and a few extra track cuts on the underside you can make quite a neat job.

If you want to take this approach then the layout for top and bottom of the board are like this:-

Fig4 & Fig5

The added extra you will spot here is an LED and resistor, this is useful to have to give you assurance that the board is connected correctly and is getting power.

If you want a larger version of this layout, all the diagrams are downloadable here:- Breakout Diagrams.zip

If you want to incorporate this sort of protection into the break out board then the neatest way to do it is by using surface mounting components.

The inexperienced are often wary of using these but it is not too difficult to cope with.

You need a good pair of tweezers and a fine tipped soldering iron.

Fig6

The ides is that the resistors are soldered across a hole in the board where the copper on both sides of the hole has been removed by a scalpel.

This is done after each screw connector apart from the two power and the ground pins.

So for example look at the G9 input and you will see the resistor as a small box and each side you will see the shading that represents a cut.

As I mentioned before larger images of this are available as a download.

The zener diodes are soldered between tracks with the side with two connectors on being the anode and the side with a single connector the cathode. The layout also includes two ground link lines.

There are where the tinned copper wire is pushed through the same hole to form a link on both sides.

You can see from the grey hole on the layout where these are. You can just about get away with 24 gauge wire but 25 to 30 AWG will be a touch easer to use.

The is a video linked at the top of this page showing how this was constructed.

Protected Version

The snag with just having the raw output available to inexperienced people is that accidents or errors in wiring often prove fatal for the GPIO pins, and while a Raspberry Pi is not very expensive to replace for a few pence you can save that money to spend on other things.

This protection circuit does not prevent the GPIO pin being used as an input or an output but it will protect them from a moderate amount of abuse. It uses a zener diode across the input. Basically this will prevent the input voltage from going any higher than 3V3 because at that voltage it breaks down and starts to conduct thus clamping the maximum voltage on the GPIO pin at 3V3.

Also is a negative voltage is applied then it acts like a normal diode and conducts preventing more than -0.7V being applied to the pin. Voltages outside this range will damage the GPIO pins. The 330R resistor will limit the current flow through the zener, dropping the excess voltage between that applied and 3V3 across it.

Thus the GPIO is protected when it is used as an input. However, it also has a role in protection when the GPIO pin is used as an output. By limiting the output current to a maximum of 10mA it will stop the GPIO pin from being overloaded. Also if the pin is being used as an input and a switch is connected between the input and ground, there is protection if the pin is accidentally made an output and the switch is grounded.

This would normally be a fatal condition but this simple resistor prevents it from happening.

This circuit also allows an LED to be connected directly to the board, but always remember to teach people that a resistor is needed in the circuit it just happens to be built in here.

There is a down side to protection, there always is, and it is that you can’t get more than 10mA from the GPIO pin and any logic you drive will only be pulled down to zero through 330R. Fortunately most logic families nowadays can cope with this.

Fig7 & Fig8

If you want to incorporate this sort of protection into the break out board then the neatest way to do it is by using surface mounting components.

The inexperienced are often wary of using these but it is not too difficult to cope with.

You need a good pair of tweezers and a fine tipped soldering iron.

The idea is that the resistors are soldered across a hole in the board where the copper on both sides of the hole has been removed by a scalpel.

Fig9 An example of two surface mount resistors soldered in place.

This is done after each screw connector apart from the two power and the ground pins. So for example look at the G9 input and you will see the resistor as a small box and each side you will see the shading that represents a cut.

As I mentioned before larger images of this are available as a download.

The zener diodes are soldered between tracks with the side with two connectors on being the anode and the side with a single connector the cathode.

Fig10 An example of a surface mount zener diode soldered along side a surface mount resistor.

The layout also includes two ground link lines.

There are where the tinned copper wire is pushed through the same hole to form a link on both sides.

You can see from the grey hole on the layout where these are. You can just about get away with 24 gauge wire but 25 to 30 AWG will be a touch easer to use.

There is a video link at the top of this page showing how this version was constructed.

The Though Hole version

For those of you for whom surface mount still fills you with dread.

We have a through hole component version.

At this time this version has not been fully tested but if you follow the same testing procedure shown in the assembly video for the surface mount version (link at the top of the page) it can be considered safe.

It is however worth noting that zener diodes are directional components and like the LED should be mounted as shown.

It turns out to be a bit bigger and the links are a bit more complex but it is probably easer to build for a novice as we have progressive diagrams.

Parts List with some guide prices from the time of writing is available Here

Stage 1 Track breaks underside

Fig11 Here we have a diagram of the underside of the board showing the inital track cuts note there have been 3 rows added top and bottom to allow space for lables.

Without label space the vero board is 24 tracks wide by 22 holes long with space for labels 28 long. 42 Track breaks

Stage 2 link wires topside

Fig12 Unlike the surface mount version the next stage is the link wires inserted from the top surface.

Notice where the line of the link wire appears to break at a hole.

Fig13 Here we see a close up section of (Fig12) this shows the three ways a joint wire can be mounted with an explanation of link wire break.

Two wires at the hole on left one right both go down through the board and are soldered to the track underneath.

The wire passes over the hole and is therfore not soldered.

A single wire terminates at a hole drops down through the board and is soldered to the trck underneath.

Stage 3 - 26 Way latch headder

Fig14 This is the topside latch fix connector for your ribbon cable there is a good section in the video for this the diagram shows some of the connectors soldered and some not but if you solder them all neatly it will not harm and you will not end up in a situation where a blank has a connection and one that should does not.

Stage 4 - Screw block connectors

Again see the video also this design has rows for the lables remember not to mount them right to the edge of the board

Fig15 Take care to mount them in the correct places noting the different positions for the three and four screw versions.

Stage 5 Resistors

Fig16 The resistors are positioned vertically and are not directional as all your resistors are of the same value you should not get them mixed up this is not true for almost any other project.

You may also notice all of the resistors connect along a track and straddle a break in the track except the one that shares the track with the LED Top Left.

So if you come to place a resistor and after insertion you notice as you come to solder it that it does not straddle a break in the track.

Then you have either placed the track cut or the resistor or both in the wrong place unless it's the one with the LED.

THIS SHOULD FLAG A CHECK DO NOT JUST MOVE THE RESISTOR

Fig17 This is a picture of a verticaly mounted resistor this is not from this curcuit but it does demonstate the method.

Stage 6 Zener Diodes and LED

Topside-Underside

Fig18 The vertical mounting goes for the zeners marked in red all except one thats mouted horizontally.

Where as it does not matter which way round the resistors go it does for the zeners and the LED.

The cathode should be the top connector, this is normally marked with a band.

The long leg (Anode) of the LED should go to the 5V line.

The short leg (Cathode) is somtimes indicated by a flat on the plastic casing of the LED.

Stage 7 TEST THIS IS IMPORTANT SEE VIDEO

This Project comes from the inventiveness of Mike Cook and the following links to his excellent support video showing the assembly of the protected version with the testing procedure applicable to all versions.

It is important that the protection is tested before it is used so that you can be sure it is working.

This requires that you use a multimeter, these can be bought for cheap even Maplins do a basic one for £7.99.

Thorough testing involves three types of test:-

1) Test for shorts.

Put the meter on continuity test, some meters bleep when there is continuity.

Put one probe on a pin of the IDC header, and touch the other probe on the (up to) five pins surrounding it.

There should be no continuity. Repeat this for all the pins on the header.

2) Test the series resistors.

Check the resistance between the screw terminal and the appropriate pin on the IDC header. This resistance should be zero or very close for the +5V,

3V3 and Ground connections and 330R (Ohms) for the GPIO signal lines.

If you do not measure this then check that the cuts in the track are made correctly and the soldered joints are good.

3) Over voltage protection.

Put your meter onto volts.

Connect the ground of a power supply with the ground of the breakout board.

Connect the negative terminal of your meter to the ground of the breakout board or power supply.

Connect the +5V line of the power supply to A screw connector GPIO pin.

Measure the voltage at the appropriate pin on the IDC header, you should see a voltage of 3v3 (three point three)

Repeat this for all the GPIO screw connectors.

Stage 8 Wrap and Ribbon

Then what ever version you use you want to label the connectors.

As mentioned earlier it is worth leaving an three rows of holes in front of the screw connectors to do this.

The labels can be self adhesive labels cut to size and written by hand or can be printed from a computer or label printing machine for a neater look.

Remember that the top row labels will need to be rotated 180 degrees so it reads correctly.

Finally before using the board it is a good idea to test it AGAIN to make sure the protection is working you do not want a smoking RaspberryPi.

First measure the resistance between each header pin and the corresponding screw output.

It should be 330R, a reading of zero or very low indicates the copper has not been cut completely where as a high resistance implies a bad joint or resistor. Then you need to test the zeners, wire up the board to an external 5V power supply through the 5V and ground connectors.

The green LED should light. Next remove the 5V line and connect it to each screw input in turn.

Then measure the voltage on the corresponding header pin and make sure it is no more than 3.9V (The V between the 3 and 9 indicates the position of the decimal point), any more suggests the zener is not soldered correctly or the ground is not correct. A voltage lower than 1V suggests that the zener has been placed the wrong way round.

Detail on assembly of ribbon cable is best generic and assembly of this is the same for all three designs.

The following image does show one key point of the cable.

Fig19 Notice how the sockets point in oposite directions relative to the cable get this wrong and the pin order is wrong.

If all is good it is time to connect it up to the Pi.

The key on the cable should be pointing into the Pi and the cable away from it.

I put a dab of white correcting fluid on the connectors to mark the correct way it should fit.

Note The latching connector on the breakout has a key block and can only be connected one way round the same is not true for the headder on your RPi.

Now power up the Pi and the green LED should light.

If not check the cable again and particularly the way it is constructed.

So now you have easy solder less access to the Pi’s GPIO ports along with a modicum of protection, ready for your exploration of the GPIO pins and their possibilities.